The invention relates to a circuit for the load-proportional adjustment of the driving current of a single-ended output transistor of a transistor amplifier for a variable load of the output transistor, the output transistor being operated in a common-emitter circuit, and the driving current being fed to the output transistor through a control branch.
In single-ended output stages in a common-emitter circuit, it must be possible to make available the driving current corresponding to the maximally expected load current.
If very large load currents are expected, the driving current can be relatively large relative to the total current drain, if the current gains are low. The driving current can be, for instance, in the range of 50% and possibly even 75% of the total current and more. This is very troublesome, especially in integrated standard circuits which are to be used while being unchanged and without external intervention, for different load currents.
This is because whle the dimensions must be laid out for the largest load currents, the same circuit is also to be used in applications with very low load currents, where the available supply currents are usually also very small.
Up to now, a Darlington output stage has usually been used in such cases, where the above-mentioned aspects with regard to the driving currents logically apply as well. Then, however, this current is one or two orders of magnitude smaller and can be neglected in comparison with those of the other stages.
It is found to be a disadvantage as compared to the single-ended output transistor, that the saturation voltage of about 0.9 V is higher by the amount of a base-emitter voltage. This however, is no longer adequate for instance, for reliably driving TTL-circuits, since this requires voltages smaller than 0.4 V.
With such a Darlington output stage, the problem of current drain is only solved at the price of less flexibility. (TTL circuits cannot be driven).
It is accordingly an object of the invention to provide a circuit for the load-proportional adjustment of the driving current of a single-ended output transistor of a transistor amplifier operated in a common-emitter circuit, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type, and in which the above-mentioned residual-voltage problem does not arise.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit for the load-proportional adjustment of the driving current of a single-ended output transistor of a transistor amplifier for a variable load of the output transistor, the output transistor having a base-emitter voltage and being operated in a common emitter circuit, having a control branch feeding the driving current to the output transistor, comprising a sensing transistor stage having an output circuit and being driven by the base-emitter voltage of the output transistor, a transistor serving as a driver transistor for the output transistor and being controlled by the control or input branch, and a coupling circuit through which current in the output circuit of said sensing transistor is feedable to said transistor serving as a driver transistor.
The above-defined construction according to the invention is based on the insight that the base-emitter voltage of the output transistor is a function of the output current and rises logarithmically with the latter. Information regarding the actually flowing output current is therefore available at the base emitter path of the output transistor without the need to intervene into the load circuit or to change the impedance situation in the load circuit by emitter resistors.
In accordance with another feature of the invention, the coupling circuit is a current reflector.
In accordance with a further feature of the invention, the sensing transistor stage is in the form of a transistor being operated in a common-emitter circuit and being connected in series with the current reflector.
In accordance with again another feature of the invention, there is provided a semiconductor crystal containing an integrated circuit, the output transistor and the sensing transistor being disposed immediately adjacent to each other in the crystal.
In accordance with a concomitant feature of the invention, there is provided a resistor connected to the emitter electrode of the sensing transistor.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in circuit for the load-proportional adjustment of the driving current of a single-ended output transistor of a transistor amplifier, operated in a common-emitter circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.